It was discovered early that particles of gold could be found in sand and gravel of stream and river beds. It was also discovered that the large difference in specific gravity between gold and sand could be used to separate the precious particles from gangue, noting that gold has a specific gravity of between 14.5 and 19.3, and sand has only a specific gravity of 2.5.
To exploit the difference in specific gravity, a gold pan now common was devised comprising a frustum, or truncated conical cylinder, with mildly inclining sides closed on its smaller end by a flat bottom. Use involves placing an amount of mineral matter in the pan with an appropriate amount of water. To separate comparatively large pebbles from the finer mineral matter, an agitation action is usually first employed that raises large matter above the fine matter. The pan is then moved in a circular motion with the pan sidewall inclined slightly below horizontal to wash away light sand material. With the water moving on the pan sidewall without excess spillage, a swirling action washes the mineral matter with the large and lighter matter being washed off of the pan.
In further washing of the matter with gentle agitation, small particles are lifted into the water to create a temporary suspension of the particles with particles of high specific gravity quickly falling back down to the pan sidewall while low specific gravity particles remain in suspension. Thus, particles are separated by specific gravity as particles of low specific gravity are suspended, carried and washed away in water. The process does not achieve a well-defined single separation but a continuum of separation, so it is necessary to repeat the process, progressively separating heavier particles from lighter particles until only the very heaviest remain.
Even the most skilled gold panner is not successful in recovering all of the gold mixed in the gangue using the traditional gold pan. Washing away low specific gravity particles also tends to wash away very small particles of high specific gravity with the sand. To improve the efficiency of the pan, various improvements have been attempted. One such improvement includes employing steps on the pan sidewall that create a pocket to capture the high specific gravity particles falling quickly out of suspension as the suspension flows laminarly over the steps. With the pan sidewal tilting slightly downwardly from horizontal, water progressively falls over succeeding steps and out of the pan carrying low specific gravity particles in suspension with it, as heavier particles fall out of suspension into the corners of the steps.
Another improvement now well-known is to have a spiral guide. wall on the pan side wall instead of concentric steps. As the pan is rotated instead of moved in a customary circular or orbital motion, small, high specific gravity particles are urged inwardly into the pan center along the guide as low specific gravity particles are washed in suspension from one spiral step to another until they fall out of the pan. High specific gravity particles that may inadvertently fall with the slurry over a guide into a more outward spiral segment are simply reprocessed as they resume their migration toward the pan center from a more distant position in the spiral.
The goal with this type of pan then is to lift the low specific gravity particles into suspension, leaving large heavy particles on the pan and washing the remainder of the slurry over the guides or steps and out of the pan.
To rotate the pan, it is normally mounted in its tilt position on a stand and rotated by a drive motor. As heavy mineral matter is separated in the pan from gangue, the pan functions more efficiently if its tilt angle is increasingly more steep. However, it is not known previous to this invetion to have a stand providing adjustment of a the pan on the stand during mineral classification to achieve a pan orientation of different tilt angles.